Side scanning sonars are known for visualising the sea bed, and consequently for visualising objects resting thereon. These side scanning sonars analyze the sea bed by decomposing it into narrow juxtaposed strips corresponding to image scan lines, and by deducing the profile of each of these strips from the echo signal returned by the strip in response to an acoustic interrogation pulse. To do this, they comprise an emitter of acoustic pulses, a receiver of acoustic energy, and signal processing means for generating an image of the bottom on the basis of the signals from the emitter and the receiver. The emitter and the receiver are installed on board a vehicle in the form of a torpedo or fish which is moved at constant speed and at constant height over the sea floor. The emitter powers two emitting transducers which are symmetrically placed in the sides of the fish and each of which has a main radiation lobe directed sideways towards the sea bottom. This lobe is narrow in the direction of fish movement and broad transversely to said direction. By means of these two emitting transducers, each acoustic interrogation pulse bathes a narrow strip of the sea bottom extending perpendicularly to the direction of fish movement with sound. The sound reaches this strip initially in a zone close to vertically below the fish, and then reaches zones which are further and further therefrom, whereby the strip is scanned from its mid-point outwardly towards its ends. Between two interrogation pulses, the movement of the fish displaces sideways the narrow strip of sea bed liable to be bathed in sound, thereby enabling a large corridor of the sea bed to be surveyed by the fish. The receiver is connected to two receiving transducers analogous to the emitting transducers and pointed towards the narrow strip of sea bed which receives the sound from the emitter. Sometimes, each receiving transducer is in the form of a multiple beam transducer comprising an alignment of unit transducers capable of separating a plurality of longitudinal zones in the echos returned by the narrow strip receiving sound from the emitter, thereby enabling said strip which receives sound from the emitter to be broadened and hence enabling the fish to be moved faster. The separation is done by suitably processing the signals from the unit transducers.
To obtain an image of the bottom with suitably high resolution, it is necessary for the system of transducers to have a very narrow aperture in the direction of fish movement, and for a given size of transducer, this leads to high frequency interrogation pulses being used, eg. pulses at a frequency of about a megahertz. These pulses hardly penetrate into the sea bed at all, and they are therefore unsuitable for detecting objects hidden therein. Further, the sheet shaped lobes of the various transducers do not minimize ground reverberation, which therefore appears as a parasitic effect when attempting to detect hidden objects.
Preferred embodiments of the present invention provide a sonar system capable of detecting objects hidden at not too great a depth in the sea floor.